1. Field of the Invention
This invention relates to an alignment optical system in a projection type exposure device, particularly, a reduction projection type exposure device used for the manufacture of integrated circuits (LSI, VLSI, etc.).
2. Description of the Prior Art
In a reduction projection type exposure device, the area for one exposure is small and therefore the so-called step exposure is carried out in which a wafer is moved for each exposure to repeat the same exposure.
Generally, there are the following two types of step exposure.
(a) The type in which a reticle and a wafer are positioned entirely separately, whereafter the wafer is fed by a predetermined step amount to thereby effect exposure (the off-axis type). PA1 (b) The type in which step exposure is effected while maintaining the relative positional relation between the pattern on a reticle and the pattern on a wafer through a projection lens (the through-the-lens type).
These two types have their own merits and demerits, and when the processed amount (through put) is chiefly considered, the type (a) is more advantageous and when the accuracy is chiefly considered, the type (b) is more advantageous.
To enhance the accuracy of position adjustment, it is necessary that the light for alignment be of the same wavelength as the exposure light. The size of the area of the pattern printed on a wafer (generally called the chip size) is not always constant. It is also necessary to reassume the relative positional relation between the pattern on the reticle and the pattern on the wafer during each exposure. This is called the step alignment. The alignment mark on the wafer used in the step alignment is usually placed on a scribe line lying immediately outside the effective area. If, during the step alignment, the other effective area than this scribe line is illuminated, it will impart other exposure than the regular exposure to the effective area and will thus cause irregularity of exposure, flare and ghost. Moreover, such an optical system for step alignment is inserted between the reticle and the condenser lens for exposure illumination and therefore, it must be disposed such that no vignetting occurs to the exposure, and this has meant great limitations.